The present invention concerns a liquid crystal display device intended, in particular, for the display of coloured images and more particularly such a liquid crystal display device of the cholesteric type allowing colours and in particular the colour red to be displayed with a high level of purity.
The characteristic feature of certain cholesteric liquid crystals is a periodic helical structure having a pitch which can be adjusted. This helical structure known as "planar " causes Bragg reflections whose reflection band, i.e. the range of wavelengths which it can reflect, may easily be changed by varying the pitch of the helix and/or the birefringence of the liquid crystal.
When a determined voltage is applied across the control electrodes, an electric field is created which transforms the helical structure of the crystal into an focal conic structure which is almost transmissive and reveals the surface situated behind the liquid crystal, which may for example be black if one wishes to produce a display on a black background as is described in the publication of D. K. Yong et al. entitled "Cholesteric reflective display: scheme and contrast"; Appl. Phys. Lett. 64, pages 1905 to 1994.
When several cholesteric liquid crystals having respectively an adjusted pitch to reflect a wavelength corresponding to a determined colour is introduced into a display device between two plates or substrates, a array of trichromatic pixels can be made, each pixel being made up for example of three sub-pixels of primary colours red, green and blue respectively. Control electrodes are provided on the inner surfaces of the plates, forming, for example, a matrix and allowing the liquid crystals to be excited locally to create selectively a colour picture element by additive mixture of the three primary colours. In the event that all the sub-pixels are in the planar helical state, i.e. reflecting, the resulting colour of the trichromatic pixels is white.
While the production of the primary colours green and blue poses no particular problem, it has been observed that the colour red obtained with this type of display device is always pale, which gives is a dull or faded appearance. This is explained by analysing the reflection spectrum of the red by a cholesteric type liquid crystal shown in FIG. 1. It will be noted that reflection of the wavelengths corresponding to the colour red by the liquid crystal is not perfect and that the reflection spectrum has significant fringes on either side of the main band, which detracts from the purity of the colour. These fringe effects have the additional drawback of being amplified by the human eye as can be seen in FIG. 2 which shows the reflection spectrum of FIG. 1 multiplied by the response of the human eye, which degrades the colour red and gives it a dull appearance.
In order to overcome this drawback, a solution has already been proposed in the work entitled "Liquid Crystal in Complex Geometries", by Taylor and Francis, published in 1996, page 257, such solution consisting of doping the liquid crystal with a dye which is intended to absorb the undesired portions of the reflection spectrum.
However this solution has drawbacks. The optical effect obtained is not optimum since it is possible that the light reflected by the liquid crystal has not hit dye molecules or has only been changed by a few dye molecules, so that the colour is not very saturated, or in other words, is not pure. Moreover, this solution requires that the liquid crystal and dye mixture is physically separated from the other liquid crystals reflecting respectively the green and the blue, in order to avoid diffusion of the colorant molecules in nearby liquid crystals of different colours.
Furthermore, the poor chemical stability of the molecules forming the dye, in particular in the presence of U.V. rays, reduces the reliability and lifespan of the display device.